Forensic experts in 2010 inspected coffins carrying the remains of 16 Hondurans who were killed in Mexico while trying to reach the U.S. border.

MEXICO CITY — Mexican cartels have operated with “near impunity” in recent years, exerting control over government authorities and entire regions and killing anyone at will, including the massacres of more than 250 migrants headed for the United States in 2010 and 2011, according to newly declassified U.S. government documents.

The findings, detailed in U.S diplomatic cables obtained by the National Security Archive, an independent research organization, further corroborate long-held suspicions that Mexican cartels, often in cahoots with corrupt Mexican authorities, have posed a grave danger to the country’s national security.

According to the cables, U.S. officials were so concerned that they rushed to support the Mexican government’s efforts to restore control, only to later privately raise doubts about the approach taken by then-President Felipe Calderón. In one document, officials expressed concern that the Calderón government’s actions were leading to “unintended consequences,” noting that the capture or killing of several top cartel leaders “has allowed less experienced and undisciplined personnel to fill the leadership vacuum, contributing to the spike of drug-related murders.”

In the case of the slain migrants, analysts said, the documents provide some new information and raise troubling questions of why more was not done then by either government to take preemptive action that might have prevented the killing of vulnerable migrants, whose only fault was traveling through a perilous region in hopes for a better life in the U.S.

In fact, far from trying to help migrants, authorities, especially immigration officials, tried to cover up the extent of the carnage, said Michael Evans, senior analyst and director of the Mexico/Migration Project at the National Security Archive.

“We learned that in the weeks and months following the 2010 massacre, Mexican officials seemed more interested in papering over the killings and minimizing the extent of the carnage than in solving the crimes,” he said.

Adam Goodman, a historian and migration scholar in Mexico City, said that the documents amount to an indictment of the Mexican government’s failure to protect migrants or to effectively investigate their killings.

“The documents bring to light the Mexican government’s utter failure to protect migrants passing through Mexico prior to the massacre and the fact that its efforts since have been ineffective at best,” said Goodman, a PhD candidate at the University of Pennsylvania. “They make clear that weak institutions and widespread corruption at the local, state and national level facilitated the rise of cartels vying for power and control over migration routes in Tamaulipas and the subsequent violence in the region.”

This week, the head of the National Immigration Institute, Ardelio Vargas, told the Mexican Senate that organized crime groups continue to control routes used by migrants and that the “damage inflicted on them is irreparable.”

On Friday, authorities in Reynosa, across the border from McAllen, Texas, announced that Mexican federal police had rescued 61 migrants who had been abducted by a criminal group. Most of the migrants were Central Americans, and the group included nine minors.

Also this week, a new surge in violence in the Tamaulipas city of Matamoros, across the border from Brownsville, shook the region. More than 16 people were killed, U.S. intelligence sources said, although sources on the ground said the death toll is higher.

U.S. intelligence sources said that the clashes are the result of a split of the Gulf cartel into two rival groups: the Cyclones, headed by a family member of former cartel leader Osiel Cárdenas, who’s in a U.S. prison, and Los Metros, headed by a man known as Perros (Dogs). The tense situation further increases the risk for migrants in the area.

The contents of the declassified documents was first reported this week in the Mexican magazine Proceso. They highlight the ongoing savagery of Mexico’s drug war, which continues to ravage regions of the country. More than 100,000 people have been killed or have disappeared since 2006, when Calderón declared he would take on cartels in an effort to strengthen rule of law in a country long hampered by weak institutions and endemic corruption.

One document warned of the growing strength of the Zetas, a group that operates in North Texas and Central America in addition to Mexico.

“The strength of the Zeta force is their ability to corrupt, kill, and intimidate, and these factors have given the Zetas the power to conduct activities throughout Mexico, and they have established a methodology to move into new territory and assert control over that geography,” the document said. “Zeta activities have evolved from drug trafficking to traditional organized crime.”

Evans said the documents were released as a result of a freedom-of-information effort by the archive and partner organizations in Mexico and the U.S. He said it’s part of a larger effort “to push for greater transparency in security and migration policy on both sides of the U.S.-Mexico border.”